Skin Structure and Function

Structure of the Skin

• The skin, also known as the cutaneous membrane, serves as a flexible yet robust barrier and is the body's largest organ
• It accounts for 15% of total body weight and varies in thickness from 0.05mm on the lips to 4.0mm on soles

Skin Surface

• The skin is generally inhospitable to microbes due to salt and sebum
• Sebum, an oily lipid from sebaceous glands, along with salt, has antimicrobial properties
• Microbes attached to epidermal cells are continuously shed as cells are pushed from basal layer
• The body replaces the epidermis' outer layers about once a month

Layers of the Skin

• The epidermis is bloodless and consists of 4-5 layers of tightly packed cells
• Basal cells in the epidermis divide continuously, pushing offspring toward the surface
• Daughter cells flatten, die, and absorb melanin
• Melanin, secreted by other cells in the basal layer determines skin color; more melanin results in darker skin
• Epidermal cells are filled with keratin, a waterproofing protein that forms nails and hairs when hardened
• Dendritic cells within the epidermis phagocytize microbes and deliver microbial antigens to lymphocytes
• Keratinocytes compose the epidermis, there is a basement membrane and a dermal epidermal junction
• A narrow multilayered structure anchors the epidermis

Dermis and Hypodermis

• The dermis is a tough, leathery structure made of loosely packed cells, connecting protein fibers, small muscles, sweat glands, sebaceous glands, blood vessels, nerve endings, and hair follicles; hairs grow through it
• It provides strength, flexibility, and support for the overlying epidermis
• The hypodermis is below the dermis and comprises fat cells and fibers
• It is not technically part of the skin and stored fats provides cushioning, insulation, and energy
• Fibers anchor the hypodermis to underlying tissues

Functions of the Skin

• Protection: Acts as a barrier against mechanical, thermal, UV, and physical injuries
• Thermoregulation: Maintains core temperature via vasodilation (heat loss) and vasoconstriction (heat retention)
• Sensation: Functions as a sense of touch organ; can cause distress in patients with skin conditions
• Immunological Surveillance: Key structures and cells facilitate immunological response with cytokines
• Biochemical Functions: Synthesizes cholecalciferol (vitamin D) in sunlight; liver converts it to calcidiol, then kidneys convert to calcitriol, the active form of vitamin D

Microbiome of the Skin

• Yeasts and bacteria reside in the epidermis, hair follicles, and sweat ducts
• The microbiome, also called microbiota, competes with pathogens for nutrients and produces growth-inhibiting chemicals
• Scrubbing reduces microorganisms but cannot eliminate them; deep-dwelling microbes recolonize the surface
• Microbiota typically grow in armpits and between legs; their waste products produce body odor

Significant Members of the Skin Microbiome

• Malassezia are lipophilic yeasts digesting sebum; mostly non-pathogenic, but can cause disease in the immunocompromised
• Staphylococcus and Micrococcus are aerobic, gram-positive bacteria tolerating 5-10% salt concentrations; Staphylococcus epidermidis is the most common
• Diphtheroids are non-pathogenic, gram-positive pleomorphic bacilli named after Corynebacterium diphtheriae
• Propionibacterium acnes can cause acne, yet competes with harmful microorganisms, occupying hair follicles and lowering skin pH with propionic acid

Bacterial Diseases of the Skin - Folliculitis

• Folliculitis: A hair follicle infection causing a red, swollen, pus-filled base which is known as a pimple or sty
• Furuncle (boil): A painful, large, raised nodular extension of folliculitis spreading into surrounding tissues
• Carbuncle: Several furuncles cluster; more common on thick-skinned areas
• Severe cases of folliculitis can trigger a fever

Pathogens and Virulence Factors

• Staphylococcus: Anaerobic, gram-positive bacteria that form spherical clusters and salt-tolerant
• They are typically found on human skin as well as in the upper respiratory, gastrointestinal, urinary, and genital tracts
• Staphylococcus epidermidis is a Staphylococcus species that accounts for up to 90% of bacteria on skin
• Staphylococcus aureus often grows in nasal passages and produces a variety of disease conditions

Staphylococcus virulance

• Coagulase: Clots blood, hiding the bacterium from phagocytes
• Hyaluronidase: Breaks down hyaluronic acid, enabling bacterial spread
• Staphylokinase: Dissolves blood clots, aiding bacterial spread
• Lipase: Digests lipids, including sebum
• β-lactamase: Confers resistance to beta-lactam antimicrobials only
• Slime layers: Inhibit chemotaxis and phagocytosis, aiding in biofilm attachment
• Protein A: Binds to IgG stems, inhibiting opsonization and complement activation
• Cytolytic toxins: Disrupt cytoplasmic membranes
• Leukocidin: Kills leukocytes
• Epidermal cell differentiation inhibitor: Compromises blood vessel linings, allowing for invasion
• Exfoliative toxin/toxic shock syndrome toxin: Proteins cause certain syndromes

Epidemiology, Diagnosis, and Treatment of Staphylococcus. aureus

• S. epidermidis thrives on skin, but seldom causes disease, acting as opportunistic pathogen in immunocompromised or when introduced via catheters/devices
• S. aureus intermittently colonizes moist skin folds, often transferred from the face via hands
• Gram-positive bacteria in pus indicate infection
• Coagulase test differentiates species
• Mupirocin and dicloxacillin are treatments
• Vancomycin treats resistant strains
• Prevention involves proper wound cleansing, aseptic catheter use, and antiseptics
• Staphylococcal lesion carriers should avoid certain professions
• Vaccine is currently being tested

Staphylococcal Scalded Skin Syndrome (SSSS)/Ritter Disease

• Outer epidermis cells separate which causes reddening and wrinkling of the skin followed by large blisters
• Skin begins near mouth and can spread across the body
• Fluid lacks bacteria and WBCs
• Area peels off in under 2 days
• Mediated by toxins from an infection site elsewhere
• IV drugs help such as semisynthetic nafcillin or oxacillin

Impetigo (Pyoderma) and Erysipelas

• Contagious disease leading to the appearance of small, flattened, red patches primarily on face and limbs
• Patches turn to oozing, pus-filled vesicles forming a thick, honey-colored, itchy crust
• When it spreads into surrounding lymph nodes there's a reddening or inflammation
• S. aureus and S. pyogenes are factors
• Requires use of antibiotics such as mupirocin and gentle cleaning to keep area uninfected

Necrotizing Fasciitis

• Often known as "flesh eating bacteria"
• Is an infection with redness, intense pain, and swelling at the infection site
• In the digestive bacteria muscle fascia it becomes distended and discolored
• Needs removal of affected tissue and intravenous broad-spectrum antimicrobial drugs to prevent more severity

Acne

• Displayed with inflammation and redness of the hair follicle
• Propionibacteria, such as acne, is almost always a small gram-positive component
• Excessive production stimulates the oil of the bacteria which causes inflammation
• Can be topically treated with drugs, UV light, and benzoyl peroxide

Cat Scratch Disease

• Fever for a few days along with prolonged malaise
• Can display localized swelling at the site which is near the lymph nodes
• Can be treated with antimicrobials such as with azithromycin

Pseudomonas Infection

• Bacterial Infection with a blue-green pigment as a symptom
• P. aeruginosa needs a series of virulent factors such as: fimbriae, adhesions, capsules etc..
• Often involves anti-bacterial use with combinations such as sulfadiazine and piperacillin

Spotted Fever Rock Mountain Spotted Fever

• Caused by number of arthropod borne rickettsias
• Signs: spotted non- itchy rash
• vector: the hard tick dermacentor
• treatable with antibiotics such as doxycycline

Cutaneous Anthrax

• Symptoms such as a localized itchy black lesion
• Protective antigen helps prevent the spores from invading
• treated with antibiotics

Gas Gangrene

• Infection caused by toxins related to Clostridium
• Characterized and identified by: Intense pain, bubbles of gas
• Treatable with antibiotics and proper cleaning of wounds

Small Pox

• Produces lesions that penetrate outer layers
• Smallpox is commonly known as being eradicated globally
• No treatment but attenuated (weakened) version is able to be given

Herpes

• Slow spreading and painful
• Transmitted via the mouth and herpes simplex virus
• Not treatable

Warts

• Benign (non-cancerous) growths of the epithelium that commonly occur
• caused by strains of human Papillomaviruses (HPV)
• Some types or strands can lead to different cancers
• Are more easily treated that bacterial or fungal infections

Chickenpox and Shingles

• Result of contracting the varicella-zoster virus(VZV)
• Chickenpox comes first, and is caused by spread of virus through respiratory droplets
• Shingles occurs after the VZV has become dormant for years, can be re-activated
• Can be diagonosed through inspection

Rubella

• Slightly swollen lymph nodes and a mild rash are symptoms
• Can be spread airborne transmission
• Some vaccinations can lead to immunity from the virus

Measles

• Virus that infects the respiratory tract
• Can spread throughout body by various vessels etc,..
• Rare but extreme cases can lead to death and blindness

Erythema Infectiosum

• Virus causing skin to turn red
• Can lead to serious symptoms in children with anemia
• Spread can last for several weeks

Roseola

• Common with high fever for a short time
• not fatal

Mycoses

• Fungal diseases
• caused by a superficial infection in upper layer due to the lack of penetration ability
• The fungal infection is opportunistic

Lyme Disease

• Can be chronic if not treated correctly and properly
• caused by bacterial infection and or tick bites
• treatment consists of antibiotics

Superficial Mycoses - Infections

• Do not penetrate living tissue, does not trigger an immune response
• Transmitted onto surface by other forms
• Treated with antifungal, topical applications

Treatment

• Topical applications or even oral medications that have antifungal and or antibiotic
• Many take months to regain a normal pigmentation
• Cutaneous Mycsoses: Infections can be prevented if wounds are properly cleaned